A high frequency switch is employed for controlling the connection between sending circuit and antenna, controlling the connection between receiving circuit and antenna, and switching the signal path of high frequency circuit in a mobile communication device such as a portable telephone, a pager and a radio apparatus for business.
The conventional high frequency switch has the capacitors 62, 64 and a diode 63 connected in series between the signal terminals 61 and 65, as shown in FIG. 8. An inductor 66 is connected to the anode of the diode 63, the other end of the inductor 66 being grounded via a capacitor 67. Moreover, a switch 68 is connected at an intermediate point between the inductor 66 and the capacitor 67, the other end of the switch 68 being connected via a resistor 69 to a power supply terminal 70. Also, the cathode of the diode 63 is connected to a strip line 71 with one end grounded.
Herein, the electric length of the strip line 71 corresponds to a quarter wavelength of a high frequency signal used. Also, the inductor 66 has an impedance to be large enough for the high frequency signal in a frequency band used. Moreover, the capacitors 62, 64 are coupling capacitors for cutting DC components of the bias current, and the capacitor 67 is a bypass capacitor for preventing high frequency signal from leaking into the power supply terminal 70.
In the above constitution, if the switch 68 is closed, an electric current supplied from the power supply terminal 70 flows via the resistor 69 to the inductor 66, the diode 63, and the strip line 71. Thereby, the diode 63 conducts in a forward direction bias, enabling the high frequency signal to be transferred between the signal terminals 61 and 65. At this time, the high frequency signal resonates in the strip line 71 grounded, so that the impedance as seen from the diode 63 becomes infinite, whereby the high frequency signal is transferred without loss. On the other hand, if the switch 68 is opened, the diode 63 is in a reverse direction bias to flow no current in a cut-off state, whereby the high frequency signal is not transferred between the signal terminals 61 and 65.
In this manner, the circuit of FIG. 8 controls the high frequency signal to conduct between the signal terminals 61 and 65 by opening or closing the switch 68, and functions as a high frequency switch.
In the recent mobile communication device, the communication systems in different frequency bands coexist along with the increasing number of users and the diversification of functions in frequency band in a specific communication system or at a specific location. To allow one mobile communication device to deal with a plurality of communication systems in different frequency bands, a high frequency switch usable in different frequency bands is required for its sending/receiving circuit.
However, in the high frequency switch with the conventional constitution, because only one usable frequency band is limitatively decided by the electrical length of the strip line, it is necessary to prepare the high frequency switch different for every frequency band to allow one mobile communication device to deal with a plurality of communication systems, resulting in a problem that the circuit becomes complex and a large size.
The present invention has been achieved in the light of the above-mentioned problems, and it is an object of the invention to provide a high frequency switch capable of coping with a plurality of frequency bands without increasing the circuit scale by providing a plurality of switches.